This invention relates in general to sensors and in particular to an apparatus and method for sensing the accumulation of soot and/or water in diesel engine oil.
During usage of a diesel engine, for example, the crankcase oil gradually experiences particle accumulation in the form of soot, which is a combustion by-product, in the combustion chamber of the engine. Portions of soot may then be transferred in small amounts to the crankcase oil. When the soot builds up to an unacceptable amount, such as a predetermined threshold percentage, the lubricating quality of the oil is diminished. When this occurs it may be necessary to change the crankcase oil whenever the soot content reaches an unacceptable value. For this purpose, it is desirable to measure the soot content percentage in the crankcase oil in order to detect the presence of the unacceptable percentage of soot.
Further, diesel engine oil is degraded by the presence of soot particles that increasingly accumulate with engine use. In addition, diesel oil is degraded as its additives are depleted and oil acidity is increased. It is becoming increasingly important for the efficient maintenance of diesel engines to have an on-board sensor that will accurately detect soot concentrations.
Microwave probes are known to be used for detecting soot in diesel engine oil such as the one disclosed in U.S. Pat. No. 4,345,202 issued to Nagy et al. Nagy discloses a microwave probe used to detect soot up to concentration levels of about five percent. This sensor used a single microwave frequency to characterize the real part of the relative permittivity for used diesel oil. From this data it was determined that the microwave probe could be used to measure the soot content of up to about five percent. However, more advanced diesel engines in today's markets may require detecting soot concentrations at levels greater than five percent. This new requirement for detecting higher soot concentrations presents problems for existing electromagnetic sensors, which have not been able to accurately measure the soot content in diesel engine oils when soot content is greater than about four percent. Above a four percent soot concentration level, various small soot particles appear to agglomerate to form large electrically lossy particles. These agglomerate particles adversely affect the ability of these sensors to accurately measure soot concentrations because of the change in the particles geometry and increase in electrical losses.
It is known that even though soot particles are conductors rather than dielectric they can increase the relative permittivity or relative dielectric constant of a dielectric fluid, such as diesel oil. This is because very small conductive particles in a dielectric form what is commonly called an artificial dielectric. Thus, the proposition that the soot content of engine oil can be measured by measuring the relative permittivity of the oil is viable provided there are no other factors affecting the oil's relative permittivity. The buildup of non-soot contaminants during engine service, oil formulations and oil additives may influence the oil's relative permittivity. If these variables significantly influence relative permittivity then that relative permittivity parameter cannot be used as a measurement of soot content. That is, non-soot constituents of engine oil can contribute enough to relative permittivity measurements to make those measurements unsuitable for a measure of soot content.